1. Field of the Invention
A method utilized in a wireless communication and communication device thereof is disclosed, and more particularly, to a method for distinguishing hybrid automatic repeat request processes in a wireless communication system and communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
Architecture of the radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer.
In the LTE system, the main services and functions of the MAC layer include error correction through Hybrid Automatic Repeat Request (HARQ), and Transmission Time Interval (TTI) bundling transmission.
For uplink scheduling with HARQ, if the PDCCH (Physical Downlink Control Channel) indicates the UE to receive a packet, but the packet cannot be decoded successfully on an Uplink Share Channel (UL-SCH), the MAC layer of the UE performs an uplink HARQ process to request a retransmission of the packet. Since the UE would not receive any retransmission packet during a signaling round trip time (RTT) of the HARQ process, a HARQ RTT Timer is thus configured. Note that, the uplink HARQ process is associated with HARQ information consisting of a New Data Indicator (NDI), a Redundancy Version (RV) and a Transport Block (TB) size.
In addition, there is one HARQ entity at the MAC layer, which maintains a number of parallel uplink HARQ processes allowing transmissions to take place continuously while waiting for the feedback on the successful or unsuccessful reception of previous transmissions. An operation of the HARQ entity for each TTI includes identifying the HARQ process associated with the TTI, if an uplink grant has been indicated for the TTI, delivering the uplink grant including HARQ information received from the eNBs to the identified HARQ process, and finally instructing the identified HARQ process to generate a new transmission or a retransmission.
Moreover, a TTI bundling operation (or so-called subframe bundling operation) is introduced to improve LTE uplink coverage without the overhead associated with L2 segmentation and the issues with ACK (Acknowledgement)/NAK (Negative Acknowledgement) errors. The UE activating the TTI bundling is allowed to transmit the same packet in consecutive TTIs/subframes. The UE in cell boundary utilizing TTI bundling transmission can reduce transmission delay. The activation and deactivation of TTI bundling transmission is done by RRC signaling message.
If TTI bundling is configured by RRC, the parameter TTI_BUNDLE_SIZE provides the number of subframes of a subframe bundle. Within a bundle HARQ retransmissions are non-adaptive and are performed without waiting for feedbacks (e.g. NACK or ACK) from previous transmissions according to TTI_BUNDLE_SIZE. A feedback for a bundle is only received for a specific TTI corresponding to TTI_BUNDLE_SIZE. A retransmission of a TTI bundle is also a TTI bundle.
Furthermore, for transmission of an uplink message containing a C-RNTI (Cell Radio Network Temporary Identifier) MAC control element or an uplink message including a CCCH (Common Control Channel) SDU (Service Data Unit) during a random access procedure, the TTI bundling does not apply.
The prior art does not appear to specify how to identify the uplink HARQ process associated with the subframe bundling operation. Moreover, the HARQ entity maintains at most 8 parallel HARQ processes in the uplink for non-subframe bundling operation, and at most 4 HARQ processes in the uplink for subframe bundling operation. This means that the number of the HARQ processes changes when the UE activate or deactivate the subframe bundling operation. However, when a transit between the non-subframe and subframe bundling operations occurs, the UE does not know which HARQ processes are kept activated and also does not know which HARQ process corresponds to which set of HARQ information received after the transit. Applying the wrong HARQ information to a HARQ process causes data transmission errors. The UE can encounter a number of issues as described below.
Issue 1: Consider the UE has 8 HARQ processes being used for uplink transmission in the non-subframe bundling operation. Since the prior art does not clearly specify how to identify a HARQ process associated with the subframe, the UE has a difficulty in distinguishing the HARQ process associated with the subframe without any means of identifying the HARQ process, and thereby is liable to use the HARQ information received from the eNBs for the wrong HARQ process. For example, when the UE receives an uplink grant with an NDI, the UE cannot distinguish which of the 8 HARQ processes the NDI is for. Since the UE cannot determine the NDI if is toggled or not, the UE may generate a transmission for an expected retransmission or a retransmission for an expected transmission.
Issue 2: When the subframe bundling operation is configured, 4 of 8 HARQ processes are used for uplink transmission and the other 4 processes are suspended according to the prior art. However, the prior art does not appear to specify which 4 processes are used for uplink transmission. Without specification, as mentioned above, the UE cannot know if the NDI is toggled and then cannot decide to transmit a new transmission or perform an adaptive retransmission.
Take an issue 3 for example. When the subframe bundle is de-configured, the prior art does not appear to specify how to resume uplink transmission for a suspended HARQ process. Without specifying which of the 8 processes is resumed first, as in issue 1, when the UE receives an uplink grant with the NDI, the UE cannot determine which process the NDI is for. Therefore, it is not known if the first transmission of the suspended process is a new transmission or a retransmission.
In addition to the issues mentioned above, a Message 3 transmission inapplicable for the subframe bundling operation can cause transmission errors when the subframe bundling operation is in-use. The Message 3 transmission is transmission of a random access procedure and can be performed with a HARQ process. The interval between retransmissions and a new transmission of the HARQ process is 8 ms (HARQ RTT) in the non-subframe bundling operation, but 16 ms in the subframe bundling operation. During the subframe bundling operation, the Message 3 transmission of the random access procedure does not apply bundling transmission according to the prior art, but still applies 16 ms HARQ RTT of the bundling transmission to the HARQ process, which delays the random access procedure completion since the Message 3 retransmission timing in the HARQ process is doubled.
Furthermore, as mentioned above, in a subframe bundling operation, the Message 3 transmission and retransmissions do not apply bundling transmission. However, if the Message 3 transmission collides with a subframe bundle transmission, the prior art does not appear to specify how to deal with this case.